Conventionally, in a reversible driver circuit for driving an electric motor, a bridge circuit including a relay in each arm of the bridge circuit was typically used. It was customary to connect a shunt resistor in the ground end of the electric motor for detecting the motor current, and monitor the state of the driver circuit by comparing the detected motor current with the pulse signal that controls the operation of the motor. Such an arrangement is disclosed, for instance, in Japanese patent laid open publication No. 07-213092 (in particular, pages 3–4 and FIG. 2).
It is also known to connect an FET between a power source and a switch for reversing the polarity of the current supplied to the motor, and to prevent the application of an excessive voltage to the motor by turning off the FET by using a breakdown of a zener diode. Such an arrangement is disclosed in Japanese patent laid-open publication No. 9-247848 (in particular, page 6 and FIG. 1).
However, according to such monitoring and detecting circuits, a failure of the circuit can be detected only after the electric current is supplied to the motor. Therefore, when the electric motor is applied to an automotive closure such as a power slide door, power window or sunroof, the user may discover that the motor cannot be controlled after turning on the electric motor, and the motor may behave in an unpredicted manner. Also, the failure in the motor driver circuit may be discovered only after the vehicle has started running. It is possible to install a fail-safe circuit to totally disable the motor when a failure of the driver circuit is detected, but it requires additional circuit elements and undesirably increases the complexity of the driver circuit.
Even when an abnormal motor current is detected, it was not possible to determine if the failure is in the electric motor or in the driver circuit. Therefore, the countermeasure that may be taken may not be proper if the cause of the failure cannot be determined. For instance, when there is any short-circuiting in the driver circuit, excessive motor current may be supplied to the motor, thereby damaging the electric motor or other circuit elements. When the failure is caused by a circuit element, the motor cannot be controlled as desired.